Here is the problem that I have heard from numerous friends when renting child carseats (granted I have never heard of anyone renting boosters but only infant seats and convertible seats): While you can reserve one with your car you have no absolute guarantee that the seat will be there when you pick up your car. I have had friends that got to their destination and found all the seats gone. They then had a mad scramble finding another rental agency with seats available and changing agencies. This happens much more frequently in areas with lots of families traveling and during peak holiday times but it does happen. This is why we always bring our own...who wants to have to sort that when you are tired and want to get started on your trip.

The other issue (and this probably applies more to infant and convertible seats than a booster) is that those seats are NEVER cleaned and there was a big article in Parents magazine a while ago where they did cultures on them and it was bad!

There is no reason a seat purchased in the US would not fit a car in Europe other than sheer size (and this would not apply to booster seats). They can all attach with belts or LATCH...it is just a matter of which one your car has. Our seat is a Recaro (purchased in US) and certified in US and EU as well as for airplane use (it says so on the sticker on the side of your seat). The certification is what they would look for if you have an accident and again it says so on the label on the side or back of the seat.

Yes you do need a booster seat for any one under 12 years. We just purchased one for our 9 year old cheap at about 8 euros.